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On 2020-06-08, Maria Van Kerkhove (WHO COVID-19 technical lead) stated:

From the data we have, it still seems to be rare that an asymptomatic person actually transmits onward to a secondary individual.

We have a number of reports from countries who are doing very detailed contact tracing. They're following asymptomatic cases, they're following contacts and they're not finding secondary transmission onward. It is very rare -- and much of that is not published in the literature.

We are constantly looking at this data and we're trying to get more information from countries to truly answer this question. It still appears to be rare that an asymptomatic individual actually transmits onward.

What Van Kerkhove states above seems to contradict a recent NEJM editorial (2020-05-28):

asymptomatic persons are playing a major role in the transmission of SARS-CoV-2.

Asymptomatic transmission of SARS-CoV-2 is the Achilles’ heel of Covid-19 pandemic control

Ultimately, the rapid spread of Covid-19 across the United States and the globe, the clear evidence of SARS-CoV-2 transmission from asymptomatic persons, and the eventual need to relax current social distancing practices argue for broadened SARS-CoV-2 testing to include asymptomatic persons in prioritized settings.

Who's correct? What does the best evidence at present say?

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    Maybe too current an event to answer. – fredsbend Jun 9 at 4:43
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    Be careful of the use of asymptomatic: sometimes is refers only to people who never get symptoms, and sometimes also includes people who are currently asymptomatic but later develop symptoms. – Jack Aidley Jun 9 at 9:50
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    A critical question to answer is how do you differentiate between someone who is asymptotic, someone who just isn't showing symptoms and someone who isn't infected? – Joe W Jun 9 at 14:23
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    The question mentions contact tracing so they're only looking at people who were very recently infected and may not be able to infect others yet. – xyious Jun 9 at 15:06
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    WHO had another press briefing on 2020-06-09 to clarify what was meant. (This should probably be an answer, but I'm too lazy to extract and summarise the relevant parts of the statement) – NotThatGuy Jun 10 at 16:28
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Disclosure up front: I am related to one of the authors of this article, which is why I'm aware of its existence in the first place.

This is an (as of yet still awaiting peer review) study of the progress of the outbreak on the cruise ship Diamond Princess.

Its findings indicate that

On the Diamond Princess 74% (70-78%) of infections proceeded asymptomatically, i.e. a 1:3.8 case-to-infection ratio. Despite the intense testing 53%, (51-56%) of infections remained undetected, most of them asymptomatic. Asymptomatic individuals were the source for 69% (20-85%) of all infections. While the data did not allow identification of the infectiousness of asymptomatic infections, assuming no or low infectiousness resulted in posterior estimates for the net reproduction number of an individual progressing through presymptomatic and symptomatic stages in excess of 15.

As said above, this research is still awaiting peer review, but it looks solid.

EDIT At the request of one commenter, the definition of 'asymptomatic' used by the paper:

One key reason for this may be that a substantial proportion of cases proceed asymptomatically, i.e. they either do not experience, or are not aware of symptoms throughout their infection but despite that can transmit to others. In this sense, asymptomatic infections differ from presymptomatic ones, which describes the part of the incubation period before symptoms develop during which onward transmission is possible.

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    If I am reading that quote correctly, a net reproductive number of 15 is incredibly high compared to what we know about the virus. This suggests the hypothesis that there was "no or low infectiousness" is likely rejectable. – Cort Ammon Jun 9 at 7:27
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    @cortammon Of course, the study also accounts for the fact that there aren't many places on a cruise ship where you can effectively self-isolate, so that would drive up the numbers, and again, the article needs more peer review, but yeah, the conclusions are alarming. – Shadur Jun 9 at 7:29
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    @Dreamer It says that the rate is almost certainly above 20%, which would not meet the criterion of "very rare" in the OP's question. – Nuclear Wang Jun 9 at 13:56
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    It's impossible to overemphasize how dramatically different the environment on a cruise ship is from the environments people typically work and live in. Aside from airplanes, submarines, and the sets of reality TV shows, there's not a lot better places to force lots of unrelated people to be in close contact for long periods of time. – barbecue Jun 9 at 15:09
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    @barbecue retirement homes and hospitals are two rather large, and extremely relevant, examples. – cpcodes Jun 9 at 18:35

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